Traditional phone service, commonly referred to as “POTS” for plain old telephone service, connects remotely-situated telephones to a telephone company central office using twisted pairs of copper wire. Traditional phone service was created to enable an exchange of voice information with other phone users using analog signal transmission. More specifically, a POTS telephone set takes an acoustic signal, which is a natural analog signal, and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency) for transmission over a copper wire pair.
As technology evolved, a need soon developed to provide for the exchange of digital data between two remotely-situated computing devices. Although the telephone network (local loop) is geared to analog, not digital, signal transmission, it was nonetheless possible to convey digital data from one location to another by encoding digital data as an analog signal, sending the analog signal over the copper wire pair, and decoding the analog signal at a remote location to retrieve the original digital data. These encoding and decoding steps are typically performed by computer modems.
Due to the fact that traditional analog voice transmission uses only a small portion of the bandwidth available with copper wire, the maximum amount of data that can be transmitted with computer modems communicating using the so-called voice channel is approximately 56 Kbps. The ability of computer modems to exchange data is constrained by the fact that the telephone company limits the bandwidth of the communication between POTS users to 4 kHz approximately.
Digital Subscriber Line (DSL) is a communications technique in which the constraint on frequencies is very much relaxed. Typically, frequencies in the range of roughly 25 . . . 1100 kHz are used for transmitting data between DSL modems. This allows concurrent use of the wire for data transmission and voice communication. The greater bandwidth available for data communication results in much greater data rates than were previously possible when only the voice channel was available. Note that in contrast with traditional 20 modems, there need to be DSL modems at both ends of the local loop, i.e. one at the subscriber premises, and one in the telephone company central office.
At present, DSL is used to provide high bandwidth communication links to homes and offices over ordinary telephone lines. But, although theoretical DSL bandwidths are high relative to conventional 56K modem technology, one or more practical considerations may significantly limit the actually achieved bandwidth to a much lower number. For example, if a home or business is located quite close to a telephone company central office, illustratively at a distance of less than half a mile, the customer may be able to receive data at rates up to 6.1 megabits per second of a theoretical 8.448 megabits per second, enabling continuous transmission of motion video, audio, and even 3-D effects. Under more typical conditions in the United States, individual DSL connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream.
The maximum range for DSL without a repeater is 18,000 feet (5.5 kilometers). As distance decreases towards the telephone company central office, the achievable data rate increases. Another factor is the gauge of the copper wire. The heavier 24-gauge wire carries the same data rate farther than 26-gauge wire. Beyond the 5.5-kilometer range, DSL service is possible if the telephone company has extended the central office local loop via one or more fiber optic cables, thus effectively reducing the length of the copper wire in the connection.
Several modulation techniques are being used by various types of DSL, although these are being standardized by the International Telecommunications Union (ITU). Different DSL modem makers are using either Discrete Multitone Technology (DMT) or Carrierless Amplitude Phase Modulation (CAP).
In the United States, several telephone companies are currently offering DSL services. But, unfortunately, many consumers are unable to take advantage of these offerings due to the fact that they are located too far from the central office, and the cost of installing extended subscriber loops via fiber optic cables is prohibitive. This problem is especially prevalent in outer-ring suburban and rural areas where homes are often situated on large tracts of land, and the nearest central office is many miles away.
Another shortcoming of DSL is the oftentimes large gap between what is promised by the telephone companies and what is actually achieved in practice. Many consumers have paid a high premium to “trade up” to DSL, only to be disappointed in lower-than-expected data transfer rates and inconsistent performance.
In view of the aforementioned considerations, an improved technique for transmitting high-bandwidth information over conventional telephone lines is needed.
Data to be communicated is represented in binary digits. A transmission medium connecting a transmitter and receiver of the system is capable of transporting signals, which can be disturbed by the environment, including signals between other pairs of communicating devices (cross talk). There also may be (regulatory) constraints on the power of the signal inserted in the transmission medium. The transmitter generates a signal, driving the transmission line with a signal equivalent to an encoded input sequence. The receiver receives the signal and converts that signal into an output sequence, which is a copy of the input sequence if no errors occur. The transmitter and receiver devices combined allow for calibration, providing a means for the devices together to compensate for certain types of distortion occurring in the transmission medium. The system must also ensure that the receiver can generate a local clock, appropriate for the reception and conversion of the transmitted signal into the output sequence.